Collecting diagnostic data from chips

ABSTRACT

A failing latch is identified on a chip including a plurality of latches with the failing latch receiving data propagated from a first set of test input latches. A diagnostic set of latches is determined which includes the failing latch and a set of related latches. The set of related latches each receives data propagated from at least one test input latch from the first set of test input latches. The set of related latches is identified from a related latches data source. One or more tests are performed on the chip and test output data is collected from the diagnostic set of latches. A related latches table may be created by tracing from a target latch.

BACKGROUND

The present disclosure relates to the field of electronic components andmore particularly to testing of electronic components.

Circuits can be prone to defects introduced during a manufacturingprocess. To test for defects, a scan input may be applied to scanchannels, and the result of the scan input that is applied to the scanchannels may be analyzed. The test may include large numbers andvariations of scan inputs being applied to scan channels of the circuit.The tests may take a great amount of time, produce large amounts of datafor analysis, and require large amounts of resources for that analysis.

SUMMARY

Disclosed herein are embodiments of a method for collecting diagnosticdata from a chip including a plurality of latches. The method includesidentifying a failing latch. The failing latch receives data propagatedfrom a first set of test input latches. The method further includesdetermining a diagnostic set of latches. The diagnostic set of latchesincludes the failing latch and a set of related latches. The set ofrelated latches each receives data propagated from at least one testinput latch from the first set of test input latches. The set of relatedlatches is identified from a related latches data source. The methodfurther includes performing one or more tests on the chip. The methodfurther includes collecting test output data from the diagnostic set oflatches. The test output data is produced during the one or more tests.

Also disclosed herein are embodiments of a method for creating a relatedlatches table for a chip including a plurality of latches. The methodincludes selecting a target latch. The method further includes tracingdata input from the target latch back to a first set of source latches.The method further includes tracing data propagation from the first setof source latches to a set of sink latches. The set of sink latchesincludes the target latch and related latches. The method furtherincludes recording the related latches in the related latches table forthe target latch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a high-level block diagram of an exemplary teststructure, according to an embodiment.

FIG. 2A depicts a high-level block diagram of an exemplary teststructure, according to an embodiment.

FIG. 2B depicts a high-level block diagram of an exemplary MISRstructure, according to an embodiment.

FIG. 3 is a flowchart illustrating a method for testing a chip,according to an embodiment.

FIG. 4 is a flowchart illustrating a method for testing a chip,according to an embodiment.

FIG. 5 is a flowchart illustrating a method for sending bits to atester, according to an embodiment.

FIG. 6 depicts a flow diagram of an example method for collectingdiagnostic data from a chip.

FIG. 7 depicts an example method for creating a related latches table.

FIG. 8 depicts an example representation of a set of source latches, aset of sink latches, and areas of logic between the latches.

DETAILED DESCRIPTION

A number of defects may be created during the manufacturing of a digitalintegrated circuit (IC). These defects may affect the logic output ofthe digital IC, which in turn adversely influences semiconductor chipquality and costs. Industry has developed a number of testing techniquesto test for the defects. These techniques may rely on a test structurebuilt into a chip. Cycling and analyzing output from the circuits on thechip may be time consuming and resource intensive. In embodiments of thepresented invention, a storage may be in communication with a scanchannel. The storage may store one or more selected bits unmodified of ascan channel during a scan out of the scan channel. In some embodimentsthe storage may be referred to as a “snapshot”. Since the organizing andanalysis of the complete test data is very time consuming the storage ofthe selected bits may reduce the number, variety, or time of testingdone to the chip. The stored unmodified bit may be analyzed while testis being completed and may be used to focus current analysis of data orfuture testing to be done.

The test structure may connect to individual scan channels. An input,sometimes known as a test vector, from the test structure may produce anoutput on the scannable latches. In various embodiments, the output ofthe scannable latches may be compared against an expected result for thecircuit being tested. The comparison may be done by an analysis systemin communication with the scan channel. In some embodiments, the teststructure may be part of a Logic Built In Self Test (LBIST) system.

FIG. 1 shows an embodiment of a test structure 100 that may be used totest a scan channel 115 on a chip. The test structure 100 may includecontroller 105 managing an input 110, or scan input data, to scanchannel 115, the output, or scan out, of which may be reviewed by ananalysis system 140. In various embodiments, a test system may bereferred to as an analysis system. The test structure may also containan aperture 130 and a storage 120. Storage 120 may store unmodified oneor more selected bits of scan channel 115 during a scan out of scanchannel 115. The aperture 130 may be defined by the controller 105 toselect the bit to store in storage 120.

In some embodiments, controller 105 may manage input 110, aperture 130,and storage 120 of test structure 100. In various embodiments,controller 105 may create and transmit input 110 to scan channel 115. Inother embodiments, input 110 may be created in another element in teststructure 100 that is in communication with controller 105. Input 110may be called scan input data in some embodiments. In variousembodiments, the input may have expected results when scanned bycircuits on a chip, such as the scannable latches 116 a-116 n of scanchannel 115. The controller may also manage storage 120. In variousembodiments this may include selecting the bit or bits to be storedduring a scan out of scan channel 115 by defining the aperture 130 whichis used on the scan out of the scan channel 115.

Scan channel 115 may include scannable latches 116 a-116 n (generallyreferred to as 116). In various embodiments the scan channel may includeany number of scannable latches 116. In various embodiments, scannablelatches 116 may be referred to as a logic element. Scannable latches 116a-116 n may be in communication with logic circuits 117 a-117 n(generally referred to as 117) on a chip. Logic circuits 117 do not haveto be of equal number to scannable latches 116. In various embodiments,logic circuits 117 may be used, assigned, or in communication withspecific scannable latches 116 or they may be shared by scannablelatches 116 of scan channel 115. Scannable latches 116 may be configuredto scan input 110 to apply it to logic circuits 117 on a chip. Thescannable latches 116 may be configured to receive the output, oroutputs, from the logic circuits 117 of the chip.

The scan out of scan channel 115 may include the transmittal of theoutputs from logic circuits 117 received by scannable latches 116 basedoff input 110, or scan input. The transmittal of the outputs may bedirected or sent to analysis system 140. In the illustrated embodiment,the output may pass through storage 120. Controller 105 may managestorage 120 and select one or more bits, depending on the size of thestorage 120, bit of the scan out of scan channel 115 to store. Invarious other embodiments, storage 120 or aperture 130 may be managed byanalysis system 140 or another entity within or in communication withtest structure 100. In various embodiments, the output may have multiplepaths, or channels, of transmittal and may not need to pass throughaperture 130 or storage 120 to reach analysis system 140.

In the illustrated embodiment, a test region 118 may be the scan output,or part of the scan output that is to be stored, or retained, in thestorage 120. For example, controller 105 may transmit an input 110 toscan channel 115. Scan channel 115 may distribute the input across logiccircuits 117 of a chip, and scannable latches 116 may be configured toreceive output from logic circuits 117. Controller 105 may manageaperture 130 such that storage 120 stores the bit, or result, from theoutput of logic circuits 117 to scannable latch 116C unmodified during ascan out of scan channel 115. In various embodiments, storage 120 mayretain any bit from any of scannable latches 116 of scan channel 115, orphrased another way, any of scannable latches 116 may be test region118. In various embodiments, storage 120 may retain bits from multiplescannable latches 117, or retain the copies of the same bit multipletimes, in multiple locations in storage 120. In various embodiments, theselected bit may be stored unmodified, such as without compressing thebit during the scan out of the scan channel.

In the illustrated embodiment, the data stored in storage 120 may beaccessed by the analysis system 140. In other embodiments, a variety ofentities are contemplated as having access to storage 120. This mayinclude, but is not limited to, entities of an LBIST system, externaltesting system, or nonvolatile data storage. The analysis system 140access to the data, or “snapshot”, in storage 120 may allow for analysisthat may direct additional testing. In various embodiments, storage 120may be a volatile or non-volatile memory. For example, storage 120 maybe a latch. In other embodiments, the storage 120 may be a shiftregister. In another example the storage 120 may be a flash memory.Storage 120 may be any element or device capable of communicating withscan channel 115 and storing the bit of data from the scan out of scanchannel 115.

FIG. 2A shows an embodiment of a test structure 200 that may be used totest a scan channel 115 on a chip. Test structure 100 may includecontroller 105 managing an input 110, or scan input data, to scanchannel 115 the output, or the scan out, of which may be reviewed by ananalysis system 140. Test structure 200 may also contain a multiplexer219 and a Multiple-input Signature Register (MISR) 220 that may be usedto store one or more selected bits of scan channel 115 during a scan outof scan channel 115. Elements and entities with matching numbers to FIG.1 may be equivalent to those described in FIG. 1 and may be consideredto act and interact in a similar way.

In this embodiment, the combination of multiplexer 219 and MISR 220 maybe used to store data, or bits, from the scan out of scan channel 115.MISR 220 may be modified to act as a signature register. Thismodification may include disabling exclusive-OR gates in the MISR 220 sothat the bits are stored unmodified. MISR 220 may typically be used tocompress data received from the scannable latches 116. MISR 220 maytypically receive data from several scan channels 115 at the same time.In the illustrated embodiment, multiplexer 219 may be used to processscan out data from test region 218 and organize is it to be entered intoMISR 220 for storage. In various embodiments, the aperture 130 may beincorporated into the multiplexer 219 or may remain a separate element.In various embodiments, multiplexer 219 may process the selected bit forstorage in MISR 220. For example, controller 105 may inhibit feedback inMISR 220 and clock selected latch values into MISR 220 as the selectedlatch positions are scanned out of scan channel 115.

In the illustrated embodiment, the test region includes scannablelatches 116B, 116C, and 116E. They are exemplary only. In practice, anyindividual or combination of scannable latches 116 may be part of thetest region stored in the scan out. In various embodiments, multiplemultiplexers 219 or MISR 220 may be available to be used. In variousembodiments, a combination of MISR 220 and other forms of storage, suchas individual latches, may be used in combination. The test region, orselected bits, to be stored during the scan may not exceed the storagecapabilities of the total storage available such as the individual MISR220 in FIG. 2B. In normal mode, the MISR may include feedback to“compress” data being scanned into it. In the test mode of the variousembodiments where the MISR 220 is used to store unmodified bits,feedback is inhibited so that the one or more bits from selectedscannable latches are not compressed.

FIG. 2B depicts a high-level block diagram of an exemplary MISR 220structure, according to an embodiment. In normal operation, the MISR 220may input a vector of test response values in parallel from a circuitbeing tested, integrate those values with a value of current state ofthe MISR 220, and leave the result in the MISR 220 for integration withthe next vector of test response values. If any single test responsevalue from the circuit turns out to be inaccurate due to a circuitfault, the state of the MISR 220 is affected such that a final result,sometimes known as the “signature”, will not be the same as an expectedresult.

Specifically, the MISR 220 shown in FIG. 2B includes a sequence of nexclusive-OR gates 232,234, and 236 that each output to a correspondingflip-flop 238, 240, and 242 respectively, where n equals the number offlip-flops in the MISR 220. Further, each exclusive-OR gate 232, 234,and 236 has an input connected to a corresponding test response valuefrom a circuit being tested 244. In the test mode of the variousembodiments where the MISR 220 is used to store unmodified bits, theexclusive-OR gates 232, 234, and 236 may be disabled so that the one ormore bits from selected scannable latches are not hashed or modifiedwhen stored.

A characteristic polynomial of a MISR 220 indicates which flip-flops inthe MISR 220 are subject to feedback. Feedback is often created in aMISR 220 to increase the number of states that the MISR can transitionthrough. For example, in FIG. 2B, FF1 and FF0, 240 and 242 respectively,could be subject to feedback by connecting a third input of thecorresponding exclusive-OR gates, 234 or 236 respectively, to an outputof FF0 242 (shown by dotted lines in FIG. 2B). In the test mode of thevarious embodiments where the MISR 220 is used to store unmodified bits,the feedback may be disabled or prevented so that the one or more bitsfrom selected scannable latches are not compressed or modified whenstored.

In various embodiments, the controller 105 may control the clock,effectively preventing inputs from the circuit 244 to the MISR 220. Thecontroller 105 may inhibit the control clock using the multiplexer 219in some embodiments. This may allow for the data to be stored in theMISR 220 without it being replaced, or lost, as testing continues.

FIG. 3 is a flowchart illustrating a method 300 for testing a scanchannel and uses a storage to store a selected bit of the scan out ofthe scan channel, according to an embodiment of the invention. Themethod 300 may start at block 301. In block 310, testing is initializedof chip elements. This testing may include one or more scan channels forcircuits on the chip. In various embodiments, the testing may beinitialized by a variety of entities internal and external to the chip.For example the testing may be initialized by or due to, but not limitedto, a programmed testing schedule, an error in output from the chipbeing detected, or an external testing device activating a testingcycle. In block 312, a determination is made if the element tested as“good”, or passed initial testing. If the element tested as “good”method 300 may proceed to block 390 and end. If the element did not testas “good”, or did not pass the initial test, method 300 may proceed toblock 314. In block 314, a test region is determined for furthertesting. In various embodiments this may include one or more scanchannels. The scan channel may be selected due to previous testingindicating that it may have a failure. The scan channel may also beselected due to a process of elimination, for example a progressive scanthrough a region or section of the chip. The continued, specific, ordirected testing may include testing of greater variety or detail thanthe initial testing in block 312.

In block 316, an aperture may be set for scanning the scan channel. Thisaperture may restrict, limit, or mask various scannable latches 116 ofthe scan channel 115 that may be involved in an individual test, testcycle, or part of a test cycle. With the aperture set the bit to bestored in storage 120 during the scan out of the scan channel 115, themethod may continue to block 318. The aperture may be set by thecontroller 105 or by another element of or in communication with teststructure 100. In block 318, the bit to be stored in storage 120 may bedetermined. In various embodiments, the bit selected may be a specificoutput from one or more scannable latches 116 of scan channel 115. Invarious embodiments, the selection of a bit to be stored may be done bythe controller 105, analysis system 140, or another element of or incommunication with the test structure 100. In various embodiments,blocks 318 and 316 may be reversed or may occur simultaneously.

In block 320, testing may begin on the chip, or more specifically thescan channel 115. The testing may include scanning input data intoscannable latches 116. The scannable latches 116 may be configured toapply the scanned input data to logic circuits 117 on the chip and toreceive output back from logic circuits 117. The output back from thelogic circuits 117 now in scannable latches 116 may now be scanned out.During the scan out one or more bits of the scan out of scan channel 115may be stored in storage 120. In block 322, the selected bit, or bits,may be stored in storage 120 during the scan out of scan channel 115. Inblock 324, the bit, or bits, stored in storage 120 may be analyzed byanalysis system 140. Concurrently, in block 330, testing may continue ofscan channel 115. In various embodiments, the analysis of the stored bitmay be used direct, modify, or select the continued testing or futuretesting of scan channel 115 or other scan channels. The method 300 maythen end in block 390.

In various embodiments, activities of method 300 may be eliminated,combined, or more numerous than illustrated. For example, in someembodiments, initial testing of block 310 and determinations of block312 and 314 may be absent. The testing and storage of a bit of the scanout of scan channel 115 may be part of a regular test, or test schedule,and not based off a failure determination.

FIG. 4 is a flowchart illustrating a method 400 for testing a scanchannel and uses a storage to store a selected unmodified bit of thescan out of the scan channel, according to an embodiment of theinvention. The method 400 may start at block 401. In block 410, the testinput is loaded into the scan channel. For example this may include thescanning in of input 110 to scan channel 115. In block 420, thecontroller 105 may check to determine if the storage 120 is empty oravailable for storing when the scan out of the scan channel 115 occurs.If the storage is not empty then the method may proceed to block 425. Inblock 425, the storage 120 may be emptied. The emptying of the storageis discussed in more detail below. The method may proceed back to block420 to confirm the availability of the storage 120.

If the storage is empty in block 420 is the method may proceed to block430. In block 430, the input is processed by scan channel 115. This mayinclude the scan out of scan channel 115. During the scan out of thescan channel 115 the method may, in block 435, determine if aperture 130is open. If the answer is “Yes” the method may proceed to block 440. Inblock 440, the aperture is open allowing the bit being scanned out to bestored in storage 120. If the answer is “No” the aperture is closed andin block 445 the bit is not stored in storage 120. Both block 440 and445 may proceed to block 450. In block 450 the method determines if thescan out is complete. If the answer is “No” the method proceeds to block430 and for the next bit of the scan out is processed through. If theanswer is “Yes” the method may proceed to block 490 where the method mayend. Method 400 is exemplary only, and it is contemplated that variousembodiments may eliminate, multiply, or rearrange steps of theillustrated method. For example the storage may be large enough to allowmore data collection when the storage is not yet empty. Also with alarge enough storage the process can continue to the next test while thestorage is being read from one or more previous tests.

FIG. 5 is a flowchart illustrating a method for sending bits to atester, according to an embodiment. In various embodiments, this may behow the storage 120 is emptied in block 425 of FIG. 4. The method 500 ofFIG. 5 may start in block 501. In block 510 the unmodified bits instorage 120 may be requested by the tester, for example analysis system140. In block 520, the unmodified bits stored may be driven to outputpins in communication with the analysis system 140 and sent to theanalysis system 140. In various embodiments, a verification of thevalidity or need for the bits stored in the storage 120 may be made. Thesending of the selected unmodified bits from the storage to the testermay be done at a tester speed. In various embodiments, the sending ofbits at tester speed may be balanced with rate of storing the unmodifiedbits in the storage 120. For example, the tester may pull or request thebits and test them at a rate balanced with the rate that bits from ascan out may be stored at. In various embodiments, the sending of thebits may occur upon the completion of the test, rather than occurringduring testing.

In block 530, the bits, or data stored in the storage may be removed orinvalidated. In various embodiments, this may result in standardizeddata or setting being inputted to the storage. For example, setting allthe latches in the storage to a value of “0” or “1”. In otherembodiments, this may result in permission being granted to or by anentity such as the controller 105 to open the aperture 130 and overwritedata existing in the storage 120. In block 540, the emptying of thestorage 120 may be confirmed. If the storage is not empty, or availableto receive bits from a scan out the method may return to block 520 andattempt to distribute, transmit, send, or remove stored bits. If thestorage is considered empty in block 540 the method may proceed to block590 where the method may end.

Embodiments of the present invention may allow for diagnostic datacollection focused on failing latches and latches related to the failinglatches. This may allow for a smaller volume of data to be collected andloaded into a test diagnostic tool. Some embodiments may allow for useof a specially configured fault model based on the failing latches.

A related latches data source may be used to identify which latches tocollect data from for processing by a diagnostic tool. The relatedlatches data source may contain a related latches table with a list oflatches, and, for each specific latch, a list of related latches whichreceive data propagated from at least one latch which propagates data tothe specific latch. In some embodiments, the table may contain anaperture ranges for each specific latch which include the relatedlatches. The table may be created once the chip is manufactured as thisinformation may be constant for the design. The information for thetable may be obtained using known tracing methods to trace paths betweenlatches.

Once a failing latch is identified, the related latches data source maybe used to identify related latches. The failing latch and the relatedlatches may be referred to as the diagnostic set of latches. A test maybe run and data may be collected from the diagnostic set of latches. Thedata collected may be loaded into a test diagnostic tool, along with thetest input data, to determine possible faults.

In another example, once a failing latch is identified, the relatedlatches data source may be used to identify an aperture range. Theaperture range may include the endpoints of a range of latches whichinclude the diagnostic set of latches. The aperture range may alsoinclude additional latches. A test may be run and data may be collectedfrom the aperture range. The data collected may be loaded into a testdiagnostic tool, along with the test input data, to determine possiblefaults.

The methods and systems described in FIGS. 1-5 may be used to obtain thedata for the diagnostic set of latches. The data from diagnostic set oflatches may be obtained during a separate test for each individual latchor during fewer tests by collecting from a range of latches usingvarious aperture ranges which mask data outside of the aperture range.For example, the aperture range may be set to obtain data for a specificlatch for each test cycle or may include multiple latches which includeat least the diagnostic set of latches. The methods described herein forobtaining data from specific latches are provided only as an example.Any method now known or developed in the future could be used.

In some embodiments, the related latches data source may further containa list of test input latches which may be used to create a fault modelto be run by the diagnostic tool. The related latches data source maycontain, for each specific latch, test input latches which feed into thediagnostic set of latches for the specific latch. Using thisinformation, a fault model may be created which is configured to findthe possible faults specific to causing the failing latch.

Referring to FIG. 6, an example method 600 for testing a chip includinga plurality of latches is depicted. At step 610, a failing latch isidentified. The latch may have been determined to be failing using anymethod. The failing latch receives data propagated from a set of testinput latches. At step 620, a diagnostic set of latches is determinedwhich includes the failing latch and a set of related latches. The setof related latches may be identified from a related latches data source.The related latches may receive data propagated from the set of inputlatches which propagated data to the failing latch. The related latchesdata source may be a data file which contains a table with relatedlatches for each latch on the chip. In some embodiments, an aperturerange may be determined. The aperture range may be identified from therelated latches data source.

At step 630, a test may be performed on the chip. The test may includeloading test input patterns into test input data latches. At step 640,test output data may be collected from one or more latches in thediagnostic set of latches. This data may be collected using any methodor structure including those disclosed in FIGS. 1-5. Steps 630 and 640may be repeated until data is collected from all of the latches in thediagnostic set of latches. At step 650, the collected data may be loadedinto a test diagnostic tool. The test input patterns from the tests mayalso be loaded into the test diagnostic tool.

At step 660, a second set of related test input latches may beidentified. The second set of related test input may be latches whichpropagate data to the diagnostic set of latches. The related test inputlatches may be identified from the related latches data source. Therelated latches data source may contain a list of test input latcheswhich propagate data to the diagnostic set of latches for each latch onthe chip. At step 670, possible faults on the chip are determined by thetest diagnostic tool using a fault model. The fault model may beconfigured based on the second set of related test input latches and thediagnostic set of input latches. In some embodiments, a total faultmodel for the entire chip may be used.

Referring to FIG. 7, an example method 700 for creating a relatedlatches table is depicted. At step 710, a target latch may be selected.At step 715, the data input may be traced from the target latch back toall source latches. At step 720, the source latches may be tracedforward to all sink latches. The sink latches may be the diagnostic setof latches for the target latch where the diagnostic set includes thetarget latch and related latches. At step 725, the diagnostic set oflatches may be recorded in a related latches table for the target latch.In some embodiments, only the related latches may be recorded.

At step 730, it may be determined whether the test input latches relatedto the diagnostic set are also being recorded in the related latchestable. If the related test input latches are not included in the relatedlatches table, the method may proceed to step 745. If the related testinput latches are included in the table, the method may proceed to step735.

At step 735, the data input may be traced from the diagnostic set oflatches to all source latches. The source latches may be the relatedtest input latches. At step 740, the related test input latches may berecorded in the related latches table for the target latch.

At step 745, it may be determined if there are more latches to completethe related latches table. If there are more latches, the method mayproceed back to step 710. If the table is complete, the method mayproceed to step 750.

At step 750, it may be determined whether the diagnostic set for eachtarget latch will be translated into an aperture range. If thediagnostic sets are not translated, the method may proceed to step 755and related latches table may be complete with a list of related latchesfor each target latch. If the diagnostic sets are translated, the methodmay proceed to step 760 and the related latches table may be completewith an aperture range for each target latch which includes thediagnostic set of latches.

Referring to FIG. 8, an example representation 800 of source latches 810a-m, sink latches 820 a-g, and areas of logic between them 830, 840, and850 are depicted. As illustrated, latch 820 d is the target latch. Areaof logic 830 represents the logic elements which feed into latch 820 d.Thus, tracing from target latch 820 d to all source latches would resultin source latches 810 f-h.

Areas of logic 830 and 840 represent the logic elements which propagatedata from source latches 810 f-h. Thus, tracing forward from sourcelatches 810 f-h to all sink latches would result in sink latches 820b-f, the diagnostic set of latches. The diagnostic set of latchesincludes target latch 820 d and related latches 820 b,c,e,f.

Areas of logic 830, 840, and 850 represent logic elements which feedinto the diagnostic set of latches. Thus, tracing from the diagnosticset of latches back to all source latches would result in source latches810 b-1, the related test input latches.

A related latch data table for example representation 800 may list, fortarget latch 820 d, related latches 820 b,c,e,f and related test inputlatches 810 b-1. In some embodiments, the related latches table mayinclude an aperture range of 820 b-f for target latch 820 d.

While the invention has been described with reference to the specificaspects thereof, those skilled in the art will be able to make variousmodifications to the described aspects of the invention withoutdeparting from the true spirit and scope of the invention. The terms anddescriptions used herein are set forth by way of illustration only andare not meant as limitations. Those skilled in the art will recognizethat these and other variations are possible within the spirit and scopeof the invention as defined in the following claims and theirequivalents.

What is claimed is:
 1. A method for collecting diagnostic data from achip including a plurality of latches, the method comprising:identifying a failing latch, the failing latch receiving data propagatedfrom a first set of test input latches; determining a diagnostic set oflatches, the diagnostic set of latches comprising the failing latch anda set of related latches, the set of related latches each receiving datapropagated from at least one test input latch from the first set of testinput latches, the set of related latches identified from a relatedlatches data source; performing one or more tests on the chip; andcollecting test output data from the diagnostic set of latches, the testoutput data produced during the one or more tests.
 2. The method ofclaim 1, wherein the related latches data source comprises a relatedlatches table.
 3. The method of claim 1, further comprising: loading thecollected test output data into a diagnostic tool; and determiningpossible faults using a fault model.
 4. The method of claim 3, whereinthe fault model is configured based on the diagnostic set of latches anda second set of related test input latches, the second set of relatedtest input latches comprising test input latches which propagate data toat least one latch in the diagnostic set of latches.
 5. The method ofclaim 4, wherein the second set of related test input latches isidentified from the related latches data source.
 6. A method forcreating a related latches table for a chip including a plurality oflatches, the method comprising: selecting a target latch; tracing datainput from the target latch back to a first set of source latches;tracing data propagation from the first set of source latches to a setof sink latches, the set of sink latches consisting of the target latchand related latches; and recording the related latches in the relatedlatches table for the target latch.
 7. The method of claim 6, furthercomprising: tracing data input from the set of sink latches to a secondset of source latches; and recording the second set of source latches inthe related latches table for the target latch.
 8. The method of claim6, further comprising: translating the related latches into an aperturerange.